10. Post Transl. Modification

Question 1

Give an example of a protein that is constitutively secreted?

Answer 1

Collagen

Question 2

Give an example of a protein that has regulated secretion.

Answer 2

Insulin

Question 3

Which cell secretes collagen in connective tissue?

Answer 3

Fibroblasts

Question 4

What is the basic unit of a collagen fibre?

Answer 4

Tropocollagen

Question 5

Describe the structure of tropocollagen.

Answer 5

3 alpha polypeptide chains arranged in a triple helix.

Question 6

What motif is characteristic of collagen?

Answer 6

Gly-X-Y

Question 7

Why is glycine so important in collagen structure?

Answer 7

It is the only amino acid with a side chain small enough to fit in the middle of the helix.

Question 8

What are the properties of collagen?

Answer 8

High tensile strength, non-compressible, non-extensible

Question 9

In addition to glycine, what is most commonly found in the X and Y positions in the chain?

Answer 9

Proline or hydroxyproline

Question 10

What type of bonds are present in tropocollagen to stabilise the structure?

Answer 10

Hydrogen bonds between alpha chains

Question 11

What types of alpha chains are present in type 1 collagen?

Answer 11

2 alpha 1 chains

1 alpha 2 chains

Question 12

Where is type 1 collagen found?

Answer 12

Skin, tendons, ligaments and bone.

90% of body collagen

Question 13

Where is type 2 collagen found?

Answer 13

Cartilage - hyaline and elastic

Fibrocartilage has both types 1 and 2

Question 14

Before entering the ER, what is the protein chain called?

Answer 14

Prepro alpha chain

Question 15

What happens to the prepro alpha chain as it enters the ER?

Answer 15

The signal peptide at the N terminal is cleaved by signal peptidase to leave pro-alpha chains

Question 16

What 3 modifications to the procollagen chains take place in the ER?

Answer 16

1. Hydroxylation of selected proline and lysine residues
2. Addition of N-linked oligosaccharides
3. Addition of galactose to hydroxylysine residues

Question 17

After modification to the pro-alpha chains, how do the 3 chains align and attach to eachother?

Answer 17

Disulphide bonds are formed at the C terminal region between the alpha chains

Question 18

What exists at the N and C terminals of each alpha chain of the pro-collagen molecules?

Answer 18

There are extra amino acids that DO NOT form a triple helix

Question 19

What modifications does the procollagen undergo in the GOLGI?

Answer 19

O-linked oligosaccharide chains are completed by the addition of glucose

Question 20

What happens to the procollagen molecule after leaving the golgi before it can become a tropocollagen unit.

Answer 20

It leaves the golgi in a transport vesicle and is exocytosed from the cell. The remaining N and C terminal sections are removed by procollagen peptidases to produce a tropocollagen unit.

Question 21

Which terminal of the procollagen contains disulphide bonds?

Answer 21

C terminal

Question 22

Where are procollagen peptidases expressed, why is this so important?

Answer 22

Only expressed extracellularly, prevent tropocollagen fibril formation from occurring inside cells.

Question 23

How is a collagen fibril formed?

Answer 23

Lateral association of tropocollagen with covalent cross-linking produces a collagen fibril.

Question 24

How does a collagen fibril become a fibre and why are they striped in appearance?

Answer 24

Aggregation of fibrils to form a fibre. There are gaps between the individual collagen molecules which makes it stripey.

Question 25

What enzyme is responsible for the covalent bonds involved in tropocollagen cross-linking?

Answer 25

Lysyl oxidase - extracellular

Question 26

What are cofactors for lysyl oxidase?

Answer 26

Vitamin B6

Cu 2+ ions

Question 27

What disease is linked to lysyl oxidase deficiency or mutation in type V collagen?

Answer 27

Ehlers - Danlos Syndrome (EDS)

Question 28

Which enzyme is responsible for hydroxylation or pro-alpha chains in the ER?

Answer 28

Prolyl hydroxylase

Question 29

What are the cofactors of prolyl hydroxylase?

Answer 29

Vitamin C and Fe2+ ions

Question 30

What is the importance of hydroxylation of procollagen in the ER?

Answer 30

It allows increased H+ bonding to stabilise the triple helix

Question 31

How can scurvy affect collagen synthesis?

Answer 31

Lack of Vitamin C prevents activity of prolyl hydroxylase, so protein contains less hydroxyproline.
Weak tropocollagen triples helices as reduced hydrogen bonding.